1. Field of the Invention
THE PRESENT INVENTION relates to a safety-belt arrangement, and more particularly relates to a safety-belt arrangement intended for use in a motor vehicle.
2. Description of Related Art
A simple safety-belt arrangement, as in widespread use, comprises a lap-and-diagonal safety-belt arrangement, (or “three-point” safety-belt) with one end of the safety-belt being anchored to part of the vehicle, an intermediate part of the safety-belt being provided with a tongue releasably engageable with a fixed buckle, and the other end of the safety-belt, which may pass through a pillar-loop, being wound on to the spindle of a retractor. The retractor incorporates a mechanism which locks the spindle of the retractor to prevent further safety-belt being paid out from the retractor when an accident or a potential accident situation is sensed. If the safety-belt is locked in this way, however, the safety-belt can, in certain circumstances, apply a very great retarding force to the occupant of the vehicle, decelerating the occupant very swiftly, and also possibly injuring the occupant.
Thus it has been proposed to provide a force limiter in a safety-belt system, the force limiter being adapted to enable a certain length of safety-belt to be paid-out, with a force limiting or energy absorbing effect, when very high forces are applied to the safety-belt.
It has been proposed to provide an arrangement in which the force limiting effect can be varied, at the start of or during an accident situation, in dependence upon the load required to be absorbed by the safety-belt arrangement. An arrangement of this type is disclosed in WO 9749583-A in which the reel of a retractor is provided with a torsion element arrangement which provides the force limiting effect. The torsion element arrangement includes an axially extending torsion bar and a co-axial torsion sleeve. The torsion bar and the torsion sleeve each terminate with a portion having a respective toothed periphery which can be releasably engaged by a respective ratchet element. In this way the level of force of the force limiting effect can be selected, by selecting either the bar, or the sleeve, or both, to provide the force limiting effect.
It is desirable, in a typical accident situation, for the force limiter to provide a high force limiting effect during the initial stages of the accident, as the occupant of the seat begins to move forwardly relative to the chassis of the vehicle, and to provide a lower force limiting effect at a subsequent stage in the accident when the occupant has moved forwardly and begins to impact with an inflating air-bag. It has, therefore, been proposed to have an arrangement in which the force level (or energy absorbing level) of the force limiter is adjusted after a predetermined period of time has elapsed following the sensing of an accident or a potential accident situation, and thus after the pretensioner has been triggered. However, an arrangement of this type suffers from various disadvantages.
Many accidents start with minor or short interaction with a first object, such as an impact with a car having a relatively low speed, or an impact with the edge of the road, or a crash barrier, with this minor or short interaction being followed, some moments later, by a severe impact with a second object, such as an on-coming high speed car, a tree or a building. In such a situation the first interaction would be sensed by the sensor provided in the motor vehicle to detect an accident or a potential accident situation. The sensor may control various safety devices within the motor vehicle, such as a pre-tensioner and an air-bag, but would also start the operation of the timer associated with the force limiter.
In an accident of this type, the force limiter would exhibit a high level of force for a period of time following the minor or short interaction, but the occupant of the vehicle may not move forwardly at all, or may move forwardly only by a very short distance during this period of time. Before the severe impact with the second object, the timer may have timed-out, and thus the force limiter would only have the lower level of force. Then, when the severe impact occurs, which will cause the occupant to move forwardly, relative to the chassis, the level of force provided by the force limiter might well be too low to absorb all of the energy of the occupant before the occupant hits the steering wheel or the dashboard.